The present invention relates generally to roofing systems. More particularly, it relates to a ballasted roofing system in which ballast material holds in place a roofing membrane. The ballast material has improved reflective properties.
Steep sloped roofs are relatively expensive to construct; accordingly, many industrial and office buildings employ low sloped roofs. Low sloped roofs often are covered with polymeric membranes to protect the underlying building from the elements. Care must be taken to maintain the integrity of the membrane to prevent the development of gaps or openings.
Often, ballast material is applied over a roofing membrane to keep it from uplifting in wind and to maintain the integrity of seams formed when two or more adjacent membrane sheets abut one another. Also, ballast material can protect the membrane from UV light degradation, which can compromise the physical integrity of the membrane. The ballast also serves to project the membrane from damage such as hail, wind-blown debris, and fire.
At least about 280,000,000 m2 of ballasted roof systems presently are in place in the United States, and another 25,000,000 m2 of such systems are installed annually.
Obviously, building owners desire to reduce the costs necessary to cool their buildings, particularly in warmer areas with a great deal of sun exposure. Additionally, existing roof surfaces are believed to contribute to the xe2x80x9cheat islandxe2x80x9d effect observed in urban areas.
Many ballasted roof systems utilize standard ballast rock, which reflects 12-20% of solar spectrum energy. Ballast rock preferably is sized to ASTM Standard No. 4 and uniformly distributed over a membrane in an amount of about 5 kg/m2. However, naturally reflective rock is practical only when available locally. Transportation costs significantly increase when the rock is quarried far from the site of the roof.
Somewhat higher reflectance can be obtained by using a natural quartzite rock. However, such quartzite does not resist mildew and, over time, catches dirt, both of which reduce its reflectance properties.
Natural color paver blocks also can be used in ballast roofing systems. These can increase the solar reflectivity up to about 34% of solar spectrum energy.
With known ballasted roof systems, solar reflectance is relatively small and heat generation by the roofing system, and thus heat absorption by the underlying building, is unacceptably high. This results in significant cooling costs for such buildings.
A solar-reflective ballasted roof system that improves the solar reflectance of the roof and significantly reduces the amount of heat transferred to the underlying building structure remains an unmet need in the art.
Briefly, the present invention provides a method for constructing a ballasted roof system that includes a roof structure covered by a roofing membrane with ballast disposed thereon. Reflective paint is applied to the ballast to improve the solar reflective properties of the roof system. The ballast material can take the form of, for example, paver blocks and/or smooth, water-worn gravel.
In a roof system of the type just described, the ballast material can have a solar spectrum reflectance of at least 35%, and preferably at least about 50%. Reflectance, as used herein, is the fraction of solar spectrum (radiation originating from the sun including ultraviolet, visible, and near-infrared radiation) that is reflected by a surface expressed as a percent or within the range of 0.00 to 1.00. By utilizing a reflective paint on the ballast material, the roof system generates less heat to be absorbed by the ballast material and the underlying building, thus reducing cooling costs. Since less heat is absorbed by the ballast material (roof surface), then the present invention contributes to a city""s reduction of the heat island effect.
Paint can be applied in a single step by, e.g., dip coating. However, prior to applying the reflective paint to the ballast material, the latter preferably is weatherized so that its receptivity to the paint is increased. Thereafter, paint can be applied in a single spraying or, preferably, in at least two separate sprayings. A first coat can be applied in one direction, and second coat can be applied in a substantially opposite (xcx9c180xc2x0) or a substantially orthogonal (xcx9c90xc2x0) direction. An advantage of such a dual coating to already-in-place ballast is that the applied paint adheres to the ballast, but separate pieces of the ballast material do not adhere to one another nor to the roofing membrane.
The present invention provides an economical method for applying a reflective surface which advantageously does not significantly increase the cost or time involved in installing a ballasted roofing system. Nevertheless, substantial cost savings to the building owner (through lower energy usage involved in cooling the building) can be realized.
The following definitions apply hereinthroughout unless a contrary intention is expressly indicated: